This invention relates to a direct rolling method for continuously cast slabs which can prevent the formation of surface cracks during hot rolling. It also relates to an apparatus for carrying out this method.
In particular, it relates to a method and apparatus for rolling hot cast slabs either immediately after casting or after slightly heating the hot cast slabs to make the temperature in the slab uniform. Such a rolling method is referred to as a direct rolling method. The present invention is particularly applicable to continuously cast Al killed steels, Si-Al killed steels, and low allow steels containing elements such as Nb or V.
In the past, a typical method of forming hot rolled steel plates involved forming a cast slab by continuous casting, allowing the slab to cool to room temperature, soaking the cooled slab in a heating furnace at a high temperature for a long period, and then performing hot rolling. However, in recent years, in order to reduce the energy consumption for hot rolling, a method referred to as direct rolling was developed. In this method, a continuously cast slab is hot rolled either immediately after casting or after slightly heating the slab to obtain a uniform temperature in the slab. In direct rolling, the steps of cooling a slab and then reheating it to a high temperature are omitted, so there is an enormous savings in energy that would otherwise be required for the reheating step. In addition, the formation of scale which results in a decrease in yield can be prevented.
However, in direct rolling, there is the problem of surface cracks, which were not a problem in the conventional hot rolling methods including soaking. Namely, according to the direct rolling process, the temperature of a cast slab which is in the process of cooling from a molten state to a solidified state does not fall below the Ar.sub.3 point, so rolling takes place immediately after Solidification in a state in which the slab contains coarse austenite crystal grains, and during the cooling process, impurities such as S, O, and p segregate and precipitate in the austenite grain boundaries. When stress is applied by hot working, cracks form along the grain boundaries, and surface blemishes (referred to below as surface cracks) are formed in the cast slab. In particular, the temperature range in which hot ductility of a cast slab decreases is 800.degree.-1200.degree. C. This coincides with the normal temperature range for hot rolling. The formation of such surface cracks is a great industrial problem and is a major impediment to the increased use of direct rolling.
Various methods are conceivable for increasing the hot ductility of cast slabs in order to prevent the formation of surface cracks during hot rolling. These methods include (1) decreasing the level of impurities in the steel, (2) refining the austenite grains, and (3) agglomerating and coarsening precipitates so as to decrease the number of precipitates at grain boundaries. A number of direct rolling methods employing these concepts for preventing surface cracks have actually been proposed.
However, these proposed methods are not without drawbacks. For example, the level of impurities can be decreased by desulfurization and dephosphorization processes during refining, but these processes unnecessarily decrease the level of S and p, leading to an increase in production costs.
In addition, austenite crystal grains can be refined by performing heavy working at a temperature higher than the temperatures at which precipitation of elements which are harmful to hot workability occurs. During such heavy working, shape control of precipitates is simultaneously carried out, and it is said that hot workability is increased. However, in a conventional continuous casting method, it is difficult from a practical standpoint to feed a hot cast slab to a rolling apparatus while maintaining the temperature of the slab at 1200.degree. C. or higher. For example, special heating equipment for preventing a decrease in the temperature of the cast slab becomes necessary, leading to an increase in equipment costs. Thus, increasing the hot rolling temperature may be impractical from a cost standpoint.
In order to aggregate and coarsen precipitates, it is necessary to maintain a cast slab for a long period in a high temperature range in which harmful elements precipitate or to perform gradual cooling in such a high temperature range. According to Met. Sci. Tech., 1 (1985), p. 111, a slab must be maintained at a constant temperature for at least 10 minutes to achieve the desired results. However, such a long holding period greatly reduces production efficiency and in many cases is impractical.
Thus, the methods which have been proposed in the past are not satisfactory from an industrial standpoint, and in order for the use of direct rolling to increase, there is a need for a more practical method.
Japanese published Examined Patent Application No. 5-68525/1993discloses a method in which a continuously cast slab is subjected to a slight degree of reduction of at most 5% and then held for 1-5 minutes prior to direct rolling. According to that method, the precipitation of harmful precipitates is in fact promoted, and the precipitates are coarsened and rendered harmless prior to the main rolling so that surface cracks can be prevented. Of the various methods which have been proposed thus far, that method is the most practical.
In recent years, however, in order to decrease costs, direct rolling has been carried out using slabs with a thickness of less than 100 mm which are cast at a fairly high speed, and in some cases the slabs are rolled to a final shape without being cut a single time. With such a direct rolling method, holding a slab for more than one minute is frequently difficult or impossible from an operational standpoint, so in this case, the method described in Japanese published Examined Patent Application No. 5-68525/1993 is unsuitable. Accordingly, these is a great need for a direct rolling method which can completely prevent surface cracks when used with thin slabs cast at a high speed.